Well, there are a lot of terms, I'll just explain some of the "popular" ones.
Wall
Walls have very high defense. They are usually used to block physical moves.
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Sweeper
Sweepers try to quickly knock out ("sweep") the opponents Pokemon. They have high Speed and Offensive stats.
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Tank
A Pokemon that can take hits, and fight back, like a a "tank"
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Staller
Like a tank, a Staller takes hits and fights back. It stalls.
More info on the terms
